It's possible to reformat an entire file, or a section of a file, using Vim's built-in = filter. Vim veterans often find this operator to be one of the most useful in their repertoire, but so common that it becomes second-nature and is rarely mentioned.

In normal mode, typing gg=G will reindent the entire file. This is a special case; = is an operator. Just like d or y, it will act on any text that you move over with a cursor motion command. In this case, gg positions the cursor on the first line, then =G re-indents from the current cursor position to the end of the buffer.

In visual mode, typing = will fix indentation of the current section. Thus, an equivalent but less efficient command to accomplish the same as gg=G in normal mode, would be ggVG to select the entire buffer in visual mode, followed by = to re-indent the entire selection.

The power of = is certainly not limited to the entire file. == will re-indent just the current line, or the = operator can be combined with text-objects for very powerful results.

If you often re-indent large areas (like the entire file, or a large text object), it can be useful to map a key to do it for you, using marks to restore your position. To format and return the same line where you were, just add this mapping to your vimrc:

map <F7> mzgg=G`z<CR>

Now, just press <F7> whenever you want to format your file.

Note, the = operator works using whatever automatic indentation settings you have turned on. If you don't have this set up, see indenting source code before using.